Back pressure fluid gaging circuit are well known in the art and, in one embodiment, consist of gage tooling equipped with an air jet supplied with fluid under a variable regulated fluid pressure. Changes in pressure upstream (the back pressure) of the jet, caused by variation in the distance between the workpiece and the gage tooling, are used to measure the distance and thus the dimensional variations in the workpiece.
While generally satisfactory, such tooling has inherent limitations. Typically, these systems require a fluid pressure regulator which is adjustable during set-up for operation to provide an output which is within the display range of the output device for the entire measuring range. Use of a variable fluid pressure regulator requires in many applications that the regulator be arranged in the measuring system in a rather specific and cumbersome orientation which dictates that the product including such a regulator be rather large and bulky. One such apparatus is shown in the U.S. Pat. No. 3,792,605 to Rabenau for "Method and Circuit for Fluid Pressure Gaging".
Other undesirable features of some prior art fluid or air gaging systems include a rather high consumption of pressurized fluid or air and a rather slow response time. Both features reduce the effectiveness of the air gaging systems for industrial applications where it is desirable to gage a large number of workpieces quickly and efficiently and with a minimum use of pressurized fluid.
Another limitation of some prior art fluid gaging systems for dimensional measurement is that they do not provide a back pressure which varies linearly over a wide range of distances (gaps) between the tooling and the surface of the workpiece or which are dependent upon the type or location of gaging tooling used. Examples of such prior art are shown in U.S. Pat. Nos. 3,178,931; 2,636,380; 2,706,397; 2,925,692; 3,621,862; 3,353,402; and 2,795,855.
Accordingly, the dimensional fluid pressure gaging circuits of the prior art have significant limitations and disadvantages.